1. Field of the Invention
The present invention relates to optical apparatuses and methods of using the apparatuses. Particularly, the present invention relates to an optical semiconductor amplifier device, which is capable of detecting whether light at a specific wavelength is amplified or not by monitoring the change in inter-junction or both-end voltage of a semiconductor laser structure, and a method of using this optical amplifier device. The present invention also relates to an optical semiconductor amplifier device, which utilizes the change in a voltage applied to a semiconductor laser structure caused by an incoming signal light in a constant current operative state and a method of using this amplifier device. The present invention further relates to an optical semiconductor amplifier device which includes a plurality of optical amplifying regions and in which at least one of the amplifying regions is a region for detecting the change in a voltage applied thereto at the time of optical amplification, and a method of using this amplifier device. Yet another aspect of the present invention relates to a tunable filtering device which is capable of changing a filtering wavelength based on a detection result of the change in a voltage applied to a semiconductor laser structure and a method of using this tunable filtering device.
2. Related Background Art
Conventionally, when a light is amplified by an optical semiconductor amplifier device, in order to detect in which wavelength range the wavelength of the light lies, a part of an incoming light prior to the incidence onto the optical semiconductor amplifier device, or a part of an amplified light, is split and the split light is input in to a detector portion which includes a wavelength detecting means for detecting the split light's wavelength.
Further, in order to perform an automatic power control (APC) amplification of the optical semiconductor amplifier device, there has been conventionally presented a system in which the change in a voltage occurring in the optical amplifier device at the time of amplification of a signal light is utilized.
FIG. 1 shows such a system. In FIG. 1, reference numeral 601 designates an incoming light, reference numeral 602 designates an optical semiconductor amplifier device, reference numeral 603 designates an emerging light, reference numeral 604 designates a control circuit, reference numeral 605 designates a current source, and reference numeral 606 designates a bias T. The incoming signal light 601 is a digital signal, and a sinusoidal wave signal whose period is sufficiently lower than the transmission rate of-the digital signal is overlapped or superposed on the digital signal. Due to this superposition, the change in the applied voltage occurring when the light signal 601 is amplified by the optical amplifier device 602 is supplied to the control circuit 604 by the bias T 606, and a control signal is supplied to the current source 605 so that the amount of the voltage change, synchronized with the frequency of the superposed sinusoidal wave, is maintained at a constant value. Thus, the APC amplification operation is achieved by changing the bias current injected into the optical amplifier device 602 from the current source 605.
In the above-discussed prior art detecting apparatus of the amplified wavelength, optical or light loss accompanies the splitting of a part of the signal light, and an optical component for light splitting is inevitably needed.
Further, the above-mentioned prior art APC amplification operation entails the following disadvantages:
When the incoming signal light travels through the optical semiconductor amplifier device the light signal is amplified due to the induced emission due to population inversion created by the current injection, to be emitted as the emerging light 603. The amplification operation is the phenomenon that accompanies the carrier recombination in an active layer or region of the optical semiconductor amplifier device. Therefore, at the time of an amplification operation, the carrier density in the active layer becomes small, compared with a non-amplification time, and the voltage produced between both opposite junction ends or the both-end voltage of the device is decreased. The amount of the voltage change grows larger as the incoming light increases and as the amplification factor increases. In the prior art structure wherein the current injection is conducted solely through two opposed electrodes and the voltage change is also detected thereby, the following drawbacks occur:
First, since the incoming light 601 is amplified and increased as this light travels through the optical amplifier device 602, the voltage change is gradually increased along a light propagation direction, and hence the amount of the voltage change is detected only in a form of its averaged value.
Second, a current flows in the light propagation direction since a voltage gradient appears in this direction, and hence the amplification factor of the entire device is lowered.
Third, since a portion for detecting the voltage change is common to a portion for adjusting the amplification factor, the above control is difficult to achieve.
Further, in a wavelength division multiplexing (WDM) optical communication system, a portion that has a wavelength selection function for selecting a light at one wavelength from lights of a plurality of wavelengths is required to be disposed in a receiver of the communication system. FIG. 2 shows an example of a WDM optical communication system. In FIG. 2, a uni-directional N to M transmission of the WDM optical communication is shown. In FIG. 2, reference numerals 700-1 through 700-N designate optical transmitters, reference numeral 701 designates a light combining device, reference numeral 702 designates an optical fiber transmission line, reference numeral 703 designates an optical branching device and reference numerals 704-1 through 704-M designate optical receivers.
In the structure of such WDM communication, when the transmission is conducted from the transmitter 700-1 to the receivers 704-J and 704-K, the receivers 704-J and 704-K are required to be capable of receiving the same wavelength. In order to achieve this operation, the wavelength selection function portion of the receivers 704-1 through 704-M is composed of, for example, an optical demultiplexer and photodetectors for respectively receiving demultiplexed lights, and the output of one photodetector is used according to the need (see FIG. 3). In the alternative, there is arranged a tunable band pass filter that is capable of changing its transmission wavelength range according to a signal from outside, and a part of the output of this band pass filter is split. The split light signal is input into an apparatus having a wavelength-detection function, and the tunable hand pass filter is controlled by the output of this wavelength detecting apparatus (see FIG. 4).
Further, in another method, an incoming signal light is branched into a plurality of portions, and the split light portions are respectively received by wavelength-fixed type band pass filters and photodetectors to achieve the same performance.
FIG. 3 shows an example of wavelength selection means which includes the above-mentioned optical demultiplexer. In FIG. 3, reference numeral 801 designates an optical demultiplexer, reference numerals 802-1 through 802-N designate photodetectors, and reference numeral 803 designates a control apparatus for selecting one of the outputs of the photodetectors 802-1 to 802-N and supplying an electric signal 805 to a terminal equipment. Reference numeral 804 designates a light signal to be input into the optical receiver 704-I.
FIG. 4 shows an optical receiver 704-I that includes the above-mentioned tunable band pass filter, and FIG. 5 shows a wavelength selection portion 901 that is contained in the receiver 704 of FIG. 4. In FIG. 4, reference numeral 901 designates a wavelength selection (WS) portion as shown in FIG. 5. In FIG. 4, reference numeral 902 designates a photodetector, reference numeral 903 designates a controller, reference numeral 904 designates a light signal, reference numeral 906 designates a control signal for determining the transmission wavelength of the WS portion 901 and reference numeral 905 designates an electric signal to be supplied to the terminal equipment. Turning to FIG. 5, reference numeral 1001 designates a tu nable band pass filter whose transmission wavelength range is changeable by a signal from outside. Reference numeral 1002 designates a light branching device, and reference numeral 1004 designates a photodetector. Reference numeral 1003 designates a controller for supplying a control signal to the tunable band pass filter 1001 based on the output signal from the photodetector 1004 and-the control signal 906 from outside. In this structure, since it is impossible to directly know the wavelength, a method is needed in which the transmission wavelength range of the tunable band pass filter 1001 is scanned over a predetermined range and the wavelengths corresponding to the wavelength multiplexed signals are identified. In this case, the order of the signals having respective wavelengths are given beforehand. In such a method, it is necessary, for example, to always transmit the wavelength multiplexed signals of all wavelengths even at the time of non-signal. Under those circumstances, the outputs of the photodetector 1004 corresponding to the respective wavelengths are obtained by scanning the transmission wavelength of the band pass filter 1001 from a shorter wavelength side to a longer wavelength side. If information of how many wavelengths are multiplexed in the signals is given one light signal of the wavelength multiplexed light signals can be taken out.
In those prior art apparatuses, the light output from the band pass filter is branched and the wavelength of its split light is detected in order to select light of a predetermined wavelength from signals of a plurality of wavelengths by controlling the transmission wavelength of the band pass filter based on the detection result. Also, there are arranged a plurality of wavelength-fixed band pass filters whose transmission wavelengths are set beforehand and photodetectors that are disposed behind the respective band pass filters, thereby achieving the wavelength selection function by electric selection.
However, the following drawbacks exist:
First, in the former case, the light signal intensity input into the photodetector is decreased since the light is split behind the band pass filter.
Second, in the latter case, the numbers of band pass filters and photodetectors necessarily amount to the number of wavelength multiplicity, and the number of components constructing the receiver increases as the wavelength multiplicity increases.
Further, in the case of the wavelength selecting means containing the optical demultiplexer of FIG. 3, the number of the needed photodetectors amounts to the number of wavelength multiplicity, and thus the same drawback as the above-mentioned latter case occurs.